It sounds simple enough: just say 'NO', but it isn't always that easy.
I have a "friend" that is easily intrigued, but also easily bored. He commits to projects out of genuine interest and curiosity, but if they take too long to complete (for whatever reason) he just gets bored, puts it off, and eventually commit to something else (sometimes he seeks these projects out, sometimes he's solicited). The problem is some of those projects involve deliverables to others, so to quit them would leave those others holding the bag.
Of course, this pattern continues and when he decides he should work on one of those projects to finally get it off his plate, all he can think about is the tons of others projects that are half done.
How can he take back control of his time?